feat: accept & and / as artist separators in metadata editor

_fmtArtists and _normalizeArtists now split on & and / in addition to
• · ; , — so "Arriez & King Dif" typed in the field normalizes correctly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui.js

@@ -240,7 +240,7 @@ document.addEventListener('click', e => {
 // Track artist → "Serph; Zun"  |  Album artist → "Serph, Zun"
 function _fmtArtists(str, sep) {
   if (!str) return '';
-  return str.split(/\s*[•·;,]\s*/).map(s => s.trim()).filter(Boolean).join(sep);
+  return str.split(/\s*[•·;,&/]\s*/).map(s => s.trim()).filter(Boolean).join(sep);
 }
 
 function _renderInfoModal(s) {